The xerographic imaging process begins by charging a photoconducting imaging member to a uniform potential, and then projecting an image of an original document onto its surface. Projection of an original document onto the charged imaging member discharges the surface in areas corresponding to light reflecting, non-image areas in the original document while maintaining charge in the dark, image areas. This selective discharging process produces an electrostatic latent image of the original document on the surface of the imaging member. Developer material is then made available to the surface of the imaging member to transform the latent image into a visible reproduction. The developer typically consists of toner particles with an electrical polarity opposite that of the imaging member, which cause them to be naturally drawn to its still charged areas. A blank copy sheet is brought into contact with the imaging member and charged electrostatically to transfer the toner image to it. After removal from the imaging member, the copy sheet is heated, thereby permanently affixing the reproduced image to the sheet. This results in a "hard copy" reproduction of the original document. The imaging member is then cleaned to remove any residual developing material from its surface and is then exposed to light to prepare it for subsequent imaging cycles.
The most common type of imaging member is called a "photoreceptor." This device is made from a belt or drum coated with a photoconducting film. When a photoreceptor is provided, light from the image is projected from the original document onto the photoconductive surface, to discharge it in areas corresponding to non-image areas in the document. Although this is the most widely used type of imaging member, other types are available.
Specifically, a pyroreceptor is a belt or drum that has been manufactured from a pyroelectric material--one that changes electrical polarization as changes in its temperature take place. In a pyrographic printing machine of the present invention, imaging data is first transmitted to some type of thermal print stylus array, which transfers heat to the pyroreceptor in a pattern corresponding to the image on the original document. This creates an electrostatic net charge in the areas at which the stylus has contacted the pyroreceptor, but leaves no net charge in the areas which were not heated. Thus, an electrostatic latent image is generated on the pyroreceptor surface which can be developed with toner which is transferred and fused to copy media for reproduction of the original image.
Some advantages to using a pyroreceptor rather than a photoreceptor include extending the period of time during which the latent image will remain stable, thereby enabling image development to take place at leisure. Also, eliminating the requirement for a high voltage device to charge the imaging member provides a significant reduction in size, cost, and ozone generation. In addition, the linearity of the pyroelectric effect (V.sub.p .about.K.sub.p .DELTA.t where V.sub.p =potential, K.sub.p =pyroelectric constant, .DELTA.t=temperature change) enables pyroelectric imaging methods to provide electrostatic latent images which produce gray level information in the form of absolute potential, or charge, levels. It is also possible to obtain precisely determined pyroreceptor surface potentials since charge is not transported through the pyroreceptor layer, as it is through a photoconductive layer. The presence and population state of traps in photoreceptors can significantly modify internal charge transport following exposure. Rapid dark discharges relative to system cycle times cause loss of contrast of the electrostatic latent image to be presented for development. Variations such as the amount of time the machine is actually in use, and photoreceptor aging effects further compound the difficulty of using xerographic photoreceptors to achieve adequate consistency in gray level imaging. Xerographic imaging is therefore usually restricted to a binary imaging format. Toner is either developed, or not developed, depending upon the magnitude of image potential (or charge) relative to a narrow threshold. The generation of gray scale output is achieved by halftone techniques which place a significant burden on system resolution requirements, and therefore, the scanning and modulation frequency requirements for digital imaging.
The following disclosures may be relevant to various aspects of the present invention:
U.S. Pat. No. 5,557,393 to Goodman issued Sep. 17, 1996 discloses a process and apparatus for achieving customer selectable colors in an electrostatographic imaging system. Among the compatible toner compositions that may be selected are toner compositions having blend compatibility components coated on an external surface of the toner particles and particulate toner compositions containing therein blend compatibility components or passivated pigments. Electrostatographic imaging devices, including a tri-level imaging device and a hybrid scavengeless development imaging device, are also provided for carrying out the described process. The processes and apparatus of the present invention are especially useful in imaging processes for producing single color or highlight color images using customer selectable colors, or for adding highlight color to a process color image produced by the same apparatus.
U.S. Pat. No. 5,185,619 to Snelling issued Feb. 9, 1993 discloses a method and apparatus for printing using a pyroelectric imaging member. The surface of the pyroelectric member is thermally exposed in a localized fashion while the surface charge is neutralized. The exposed surface of the imaging member is then cooled to generate an electrostatic latent image thereon. This latent image is developed with charged toner particles, which are transferred from the pyroelectric member to a substrate through the use of a second uniform thermal treatment which serves to reverse the net charge polarity of the imaging member and thereby reduces the electrostatic forces attracting the toner particles to the imaging member. The transferred toner image may be simultaneously or subsequently fixed to the substrate by a thermal or other well known fusing treatment.
U.S. Pat. No. 5,153,615 to Snelling issued Oct. 6, 1992 discloses a method and apparatus for printing including the use of a pyroelectric material in a novel fashion to directly mark an image on a print substrate. The image is produced by initially coating a poled pyroelectric material with a uniform coating of charged marking particles and subsequently thermally exposing the pyroelectric material in a localized fashion, thus reversing the polarity of the charge which repels the particles from the surface of the pyroelectric material, and onto the surface of a print substrate placed in close proximity thereto. Subsequently, the image formed by the transferred marking particles is fixed to the substrate by a thermal or other well known fusing treatment.
U.S. Pat. No. 5,537,198 to Jackson issued Jul. 16, 1996 discloses a method and apparatus for forming color images using "double split" recharging. A recharging step takes place between two image creation steps to recharge a charge retentive surface to a predetermined potential. Pursuant to forming the second of two images, a first corona generating device recharges the charge retentive surface with a direct current to a higher absolute potential than a predetermined potential. A second corona generating device recharges the charge retentive surface with a direct current to a lower absolute potential than the predetermined potential, and then a third corona generating device recharges the surface, also with a direct current, to the predetermined potential. An electrical charge associated with the first image is prevented from reversing its original polarity after being recharged by the third corona generating device, thereby preventing the occurrence of undercolor splatter of the toner image.
U.S. Pat. No. 4,078,929 to Gundlach issued Mar. 14, 1978 discloses a method for making electrostatic charge patterns and for developing such charge patterns in two colors. A charge pattern is generated with a single polarity but to at least three different levels of potential. The charge pattern is developed in two colors by utilizing relatively negatively charged toner particles of one color and relatively positively charged toner particles of a second color.
All of the references cited herein are incorporated by reference for their teachings.
Accordingly, a need remains for improved methods and apparatus for reproducing color images using a xerographic process. Further, there is a need for processes and apparatus for creating multiple latent images which can be superimposed on top of each other prior to transfer of the developed image to copy media.